vect: Reset using_select_vl_p before starting over [PR123074].

In the PR we ICE accessing the loop lens in
vect_get_loop_variant_data_ptr_increment when building 510.parest.
This function only gets called when we use SELECT_VL for the loop
control, however during initialization of the loop lens partial
vectors, a prerequisite for SELECT_VL, was false while using_select_vl
was true.  We reset using_partial_vectors when restarting the analysis
after forcing single-lane SLP but don't reset using_select_vl.

This patch resets it as well.

	PR tree-optimization/123074

gcc/ChangeLog:

	* tree-vect-loop.cc: Reset LOOP_VINFO_USING_SELECT_VL_P.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/rvv.exp: Include *.C.
	* gcc.target/riscv/rvv/autovec/pr123074.C: New test.

0 files changed, 0 insertions, 0 deletions